Certain alpha-olefin inter-polymers have been used in adhesive compositions which should yield a significant bond strength after application, show good paper adhesion (e.g. fiber tear on Kraft paper), minimum peel strength of 500 g/cm, low color, low odor, and good thermal stability. For PSA applications, when the substrate is an OPP tape, the rolling ball tack test should yield a maximum of 3 cm at ambient temperature, a S.A.F.T. minimum value of 85° C., a shear (12.5 mm×25 mm area under a 1 Kg weight) on cardboard at 40° C. of at least 30 hours. Most known alpha-olefin inter-polymers in such compositions have a high melting point and/or a high crystallinity. These characteristics prevent such materials, on their own, from being used as an adhesive because an adhesive must a low crystallinity for flexibility and a low plateau modulus, as well as a low viscosity in many applications. (see J. Adhesion Sci. Technol. Vol 3, No 7 pp551–570 (1989) where an SBS block-copolymer is used).
In such prior art adhesive formulations, the alpha-olefin inter-polymers contribute to the bond-strength, but tackifiers are used to increase the Tg for good bond strength and bring the high plateau modulus down to an acceptable level by decreasing the polymer chain entanglements. Flow promoters (waxes etc) are used to improve the flow and ensure wetting of the substrate by the formulation. Without tackifiers and flow promoters, such inter-polymers can be used to heat seal at reduced temperatures but are not, generally, regarded as adhesives.
The inter-polymers are derived predominantly from ethylene or propylene (For ethylene based polymers see WO97-15636-A1; WO99/24516; WO9803603 (EP-912646) by way of example using single site catalyst; WO97-15636-A1 or WO94/10256; U.S. Pat. No. 5,317,070 and WO94/04625, using syndiotactic polypropylene as the polymer component, and mentioning on page 7 line 14 of hexene as comonomer. For propylene based polymers further see EP-318049. For basic monomers other than propylene or ethylene, see for example EP-422498 a butene-propylene inter-polymer with up to 20 wt % propylene derived units).
As an example of the inter-polymers used for heat sealing or impact modification, reference is made to JP62-119212-A2. This discloses a random copolymer with from 40–90 mole % of propylene, from 10–60 mole % of an alpha-olefin such as butene, hexene, and 4-methylpentene using a metallocene type ethylene-bis tetrahydro-indenyl zirconium dichloride as a catalyst. Similarly JP62-119213-A2 discloses a random copolymer of butene (60–98 mole %) with 2–40 mole % of C3-20 alpha-olefin such as propylene, hexene, and 4-methylpentene.
However, the Examples in JP62-119212-A2 have widely varying characteristics. Example 6 polymerizes propylene and hexene to give 60 percent of units derived from propylene and 40 mol % of units derived from hexene. The crystallinity is 26% and the melting point is 123° C. Example 3 uses propylene at 45 mol % with a melting point of 50° C. and a crystallinity 7%. JP62-119212-A2 does not disclose a polymer having a combination of structural characteristics (molecular weight; comonomer content for example) such that a storage modulus G′ suitable for adhesive applications is reached below 70° C. or providing a low melting peak. The polymers are said to have anti-blocking characteristics and are of no use in adhesive applications.
WO99/67307 discloses a terpolymer comprising predominantly propylene derived units for use as films, fibers, and molded articles, and also seal layers. The polymers in Table 4 have low comonomer contents, high melting points and high molecular weights.
WO9920644 discloses elastic composition of propylene homopolymers for adhesive application. Metallocenes are used in the polymerization.
In other documents, alpha-olefin inter-polymers are prepared using a conventional Ziegler-Natta catalyst with a titanium chloride transition metal component and an aluminum alkyl activator to give a polymer with a monomer composition in which the amounts of propylene (lower molecular weight comonomer) and higher molecular weight comonomer are approximately equivalent have been suggested for adhesive application. These have been referred to as A(morphous) P(oly) A(lpha) O(lefin), APAO's for short.
U.S. Pat. No. 3,954,697 discloses in example 1 a propylene-hexene-1 copolymer containing 43 mol % of hexene-1 derived units which can be coated onto a tape hot to give a pressure sensitive adhesive material. The polymer may be used without additives (See column 2 lines 34 to 39) and can be applied as a hot melt to a tape without solvent to show PSA behavior. In U.S. Pat. No. 3,954,697, the amount of hexene deemed essential for a polymer is in excess of 40 mol % and the polymer structure must be such that that the polymer is entirely amorphous and has no residual crystallinity (See Column 3 lines 24 to 26) or crystallinity of a very low order (See column 4 line 8). For example, comparative Example 9 uses 18 mol % hexene in the polymer and obtains a melting point of 145° C. suggesting the absence of rheological characteristics or melting points associated with satisfactory adhesive behavior. This polymer lacks tackiness at ambient temperature.
High propylene content APAO with butene comonomer have been made and sold under the Registered trade name Rextac using non-SSC type catalysts. WO98/42780 discusses the use of such polymers in adhesive compositions.
More details on such inter-polymers and their use in adhesive compositions can be found in U.S. Pat. No. 5,478,891. U.S. Pat. No. 5,723,546 uses blends to obtain the desired characteristics. Details can be derived from U.S. Pat. No. 4,217,428, U.S. Pat. No. 4,178,272 and U.S. Pat. No. 3,954,697 which recommend generally high amounts of the higher molecular weight alpha-olefin comonomer.
WO9823699 and EP 620257 disclose a polymer in which from 70 to 99 mol % is derived from a C6 to C12 alpha-olefin and the remainder is a lower alpha-olefin. The exemplified combinations are of hexene-propylene and octene-ethylene inter-polymers prepared with a conventional Ziegler-Natta catalyst. A low Tg can be obtained. The material may be cross-linked to improve cohesive strength. Nevertheless there are drawbacks associated with such polymers and their application in adhesive end uses. Such known APAO's are non-homogenous, have significant levels of extractables and unsatisfactory physical properties, including low cohesive strength, that restrict the application and adhesive performance.